1. Field of the Invention
The present invention relates to a steering force controller for a power steering apparatus and particularly, to a steering force controller which electronically controls steering force in accordance with vehicle speed, steering rotational angle and so forth.
2. Discussion of Background
Recently, there has been developed a power steering apparatus utilizing a microcomputer. In this apparatus, a characteristic map of a predetermined control pattern is stored in a memory, and control inputs such as vehicle speed and steering rotational angle are input to the microcomputer. The microcomputer appropriately controls the steering force generated by a power cylinder, in accordance with these control inputs and the characteristic map.
For example, there is known an electronically controlled power steering apparatus of the type wherein the steering rotational angle is detected by providing an encoder secured to a steering shaft and by causing a counter to count up or down in response to the output of the encoder and in dependence upon the steering direction. In this apparatus, as the content of the counter for storing the steering rotational angle is lost due to an electric power loss, a steering rotational angle detected when the electric power is subsequently restored coincides with that as counted from the position where the steering is located at that time. Accordingly, it is necessary to detect a neutral position of the steering again and then, to modify the previously detected steering rotational angle to a steering rotational angle as counted from the neutral position. For example, in a detecting method utilizing the fact that most of the time the steering wheel is in a neutral position, the vehicle must run a considerable distance in order that the straight driving state or the neutral position of the steering can be precisely detected by sampling the output of the encoder each time of a unit driving distance or each unit driving time and by software processing the sampled data. Thus, the steering rotational angle so detected is unreliable at such a time that the vehicle has just begun to run after the supply of electric power. If the power assist were controlled based upon a signal indicative of a steering rotational angle which is detected in an early stage of driving, the operator would suffer a curious or unpleasant feeling.